The present disclosure relates generally to glass sheets and more specifically to a glass soot deposition and sintering process for forming high surface quality glass sheets and ribbons of glass. Such glass products can be formed free or substantially free of surface particulates.
Glass sheet materials can be formed using a variety of different methods. In a float glass process, for example, a sheet of solid glass is made by floating molten glass on a bed of molten metal. This process can be used to form glass sheets having uniform thickness and very flat surfaces. However, float glass processes necessarily involve direct contact between the glass melt and the molten metal, which can lead to undesired contamination at the interface and less than pristine surface quality. In order to produce high quality float glass sheets with pristine surface properties on both major surfaces, float glass is typically subjected to one or more surface polishing steps. This processing adds additional expense. Moreover, it is believed that the float process has not been used to make rollable (i.e., very thin) glass sheets.
An additional method for forming glass sheet materials is the fusion draw process. In this process, molten glass is fed into a trough called an isopipe, which is overfilled until the molten glass flows evenly over both sides. The molten glass then rejoins, or fuses, at the bottom of the trough where it is drawn to form a continuous sheet of flat glass. Because both major surfaces of the glass sheet do not directly contact any support material during the forming process, high surface quality in both major surfaces can be achieved. Due to the dynamic nature of the fusion draw process, however, the number of glass compositions suitable for fusion draw is limited to those that possess the requisite properties in the molten phase (e.g., liquidus viscosity, strain point, etc.). Further, the apparatus used in the fusion draw process can be expensive.
Thin, rollable glass sheets having a thickness of 200 microns or less can be made using a glass soot deposition and sintering method. In such a method, particles of glass soot are formed via gas-phase reactions and deposited onto a rotating drum to form a soot layer that is released from the drum as a self-supporting soot sheet that is in turn sintered to form a glass sheet. During the act of forming the soot sheet, rogue soot particles made during the soot forming and deposition processes and not incorporated into the glass soot layer may be later incorporated into a surface of the soot sheet and undesirably incorporated into a surface of the resulting glass sheet.
Approximately 50% of the formed soot particles are homogeneously incorporated into a soot layer that is released from the deposition surface to form the soot sheet. While many of the excess particles can be collected by an exhaust system located in the vicinity of the deposition zone, a potentially significant and deleterious volume of soot particles can deposit onto the soot sheet. Such particles, rather than being homogeneously incorporated into the soot sheet, alight and spall onto the soot sheet and ultimately manifest as surface defects in the resulting glass. It can be problematic to use a high volume or high velocity exhaust system in the vicinity of the soot sheet because the gas velocity generated by the exhaust, while effective at directing soot particles away from the soot sheet, can act on and tear or rip the soot sheet.
In view of the foregoing, because thin defect-free glass sheets are desirable, it would be desirable to develop a robust process to form soot sheets that are free or substantially free of unwanted surface particles and defects caused by such particles.